Protective effect of docosahexaenoic acid on oxidative stress-induced apoptosis of retina photoreceptors

PURPOSE. In a recent study, it was demonstrated that docosahexaenoic acid (DHA) promotes the survival of retinal photoreceptors in vitro, delaying apoptosis. However, lipid enrichment in DHA is known to contribute to retina vulnerability to oxidative stress. In this study, the effect of oxidative damage on rat retina neurons in vitro and whether DHA enhances or diminishes this damage were investigated. METHODS. Rat retina neurons in 3-day cultures, with or without DHA, were treated with the oxidant paraquat. After 24 hours, apoptosis, mitochondrial membrane integrity, and Bcl-2 and Bax expression were immunocytochemically determined. RESULTS. Paraquat induced apoptosis in amacrine and photoreceptor neurons, major neuronal types in the culture. Neuronal apoptosis was accompanied by mitochondrial membrane depolarization, an increase in the amount of photoreceptors expressing Bax, and a decrease in those expressing Bcl-2. Addition of DHA reduced photoreceptor apoptosis by almost half, simultaneously preserving their mitochondrial membrane integrity. DHA blocked the paraquat-induced increase in Bax expression and remarkably upregulated Bcl-2 expression. Glia-derived neurotrophic factor, a photoreceptor trophic factor, only slightly increased Bcl-2 expression and did not protect photoreceptors from oxidative damage. Similarly, other fatty acids tested did not prevent photoreceptor apoptosis. CONCLUSIONS. These results show that oxidative damage induces apoptosis in retinal neurons during their early development in culture and suggest that the loss of mitochondrial membrane integrity is crucial in the apoptotic death of these cells. DHA activates intracellular mechanisms that prevent this loss and by modulating the levels of pro- and antiapoptotic proteins of the Bcl-2 family selectively protect photoreceptors from oxidative stress.